Variable air volume (VAV) ventilation systems provide conditioned air from a central source for distribution to various zones within a building via a network of ducts. The amount of heating and cooling provided to the various zones is controlled by varying the volume of conditioned air provided to each zone. Since heating and cooling requirements vary from zone to zone and within individual zones depending upon factors such as solar load and the nature of zone usage, it is necessary that the amount of conditioned air supplied to a zone in response to local demand be selectively controlled.
In a variable air volume system, the selective delivery of conditioned air to a zone is accomplished through the association of an air distribution box with each zone. Such distribution boxes typically include a supply plenum and one or more air outlets in flow communication with the zone. Additionally, each box has a valve for controllably varying the volume of air delivered to its plenum and to the zone. Such air valves are thermostatically controlled so as to supply the volume of conditioned air, typically through a room diffuser, necessary to maintain or achieve a selected zone temperature.
The use of electric motors for controlling the damper position in VAV air valves is known. Earlier types of electric motor driven air valves are disclosed in U.S. Pat. Nos. 4,082,114; 4,775,133 and 4,884,590, which are all assigned to the assignee of the present invention. The valve of the '114 patent includes a close-ended cylindrical portion downstream of the valve inlet in which a generally tubular member is disposed for movement axially of the valve housing. The size of a series of radial ports, and therefore the flow of air through the valve, is determined by the position of the valve member within the valve housing.
U.S. Pat. No. 4,775,133 discloses an electric motor driven air valve having a cylindrical inlet section and a physically moveable, spring biased backplate on which a drive motor is disposed. The inlet section defines a seating surface upstream of which a support grid is disposed. The spring loaded backplate is supported by a plurality of rods that extend downstream from the inlet section.
A damper assembly includes a splined rod which extends upstream of the damper and into a cooperating spline in the support grid. A threaded spindle extends downstream of the damper plate and is penetrated by a cooperatively threaded gear which is mounted for rotation on the backplate and operably coupled to the threaded spindle. The spring loading of the back plate prevents the binding of the damper drive train after the damper has been driven to the fully closed position.
The invention disclosed in the '590 patent is an air valve having a cylindrical inlet section which defines a seating surface upstream of a support grid. A fixed backplate, upon which a drive motor is mounted, is supported by a plurality of rods that extend downstream of the inlet section. The damper assembly includes a splined rod which extends upstream and into a cooperating spline in the support grid. A threaded spindle extends downstream of the damper plate through a threaded motor driven drive gear mounted on the backplate. A strain sensing device is employed in controlling the operation of the drive motor to prevent binding of the damper drive train on valve closure.
An alternative room air diffuser incorporating fire/smoke protection apparatus is disclosed in U.S. Pat. No. 4,800,804. The '804 apparatus, which combines the function of an air valve, a room diffuser and a smoke damper is relatively very complex and expensive. The drive mechanism consists of a motor and associated gear train. The gear train, associated axles and gear case necessitate upstream support structure and a plurality of guide posts and associated guide sleeves. The rod on which the damper rides and the plurality of guide posts are supported at both of their respective ends making a backplate and further associated support structure necessary.
The need for the support structure employed by conventional air dampers such as those referred to above results from the need to mount a relatively massive and/or sophisticated drive mechanism, consisting of a drive motor, gear train and supporting members, directly on a valve damper or on a backplate-like structure. With respect to the damper, compounding the problem caused by the disposition of the drive system on it is the fact that the drive systems are typically asymmetrically mounted and their weight creates a tilting moment in the damper or the need for a relatively substantial support structure to counteract the tilting moment. The damper will tend, otherwise, to bind on the guide structure as the damper translates on its central shaft.
The need continues to exist for an electric motor driven air valve, particularly adapted for use in VAV air distribution boxes upstream of a room air diffuser, which is relatively uncomplicated, lightweight, inexpensive of manufacture and easy to maintain yet which provides for precise control of the volume of air flowing through it to the distribution box it supplies and to the associated room diffuser.